Loading...
HomeMy WebLinkAbout02102004 BSC Agenda Item 2 • F • PA-SC-04-0 Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Website Publishing Foundation Performance Association—Structural Committee Page 1 of 9 RECOMMENDED PRACTlCE FOR GEOTECHNIC L. EXPLORATIONS AND REPORTS by The Structural Committee of The Foundation Performance Association Houston, Texas Document # FPA-SC-04-0 ISSUE HISTORY (Only includes issues outside the Structural Committee) Rev Date Description Subcommittee Subcommittee Chair Members D 19 Jul 00 Issued for Comments to FPA Ron Kelm Jon Monteith Geotechnical Engineers George Wozny E 21 Aug 00 Issued for Comments to Local Michael Skoller Geotechnical Engineers Moyeen Hague H 4 Jan 01 Re-Issued for Comments to Local Geotechnical Engineers J 1 Mar 01 Issued for FPA Peer Review 0 11 Apr 01 Issued for Website Publishing • FPA-SC-04-0 Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Website Publishing Foundation Performance Association—Structural Committee Page 2 of 9 PREFACE This document has been developed by a group of Structural Engineers in southeast Texas with the goal to attain the geotechnical information they believe is necessary to adequately perform their structural designs. Their need for this document has been prompted by a large number of residential and light commercial foundation problems, some of which have been the subject of litigation. As a result, this document has been prepared specifically for the Structural Engineers' use. However, it is made freely available to the public through the Foundation Performance Association at www.ffoundationoerformance.arg so others may have access to the information and may adapt it to their work as they see fit. To ensure the document remains as current as possible, it is intended to be periodically updated under the same document number but with new revision numbers. This document is a recommended practice only and is therefore intended to be neither comprehensive nor a substitute for engineering judgment or for local or standard codes and practices. The user should recognize that there is always the possibility this recommended practice might not be fully adaptable to the site being investigated and in those cases, the use of engineering judgment will be paramount. The intent of this document is to detail certain minimum requirements recommended for the geotechnical exploration and report, thereby ensuring that the Structural Engineer receives the information needed to perform an adequate foundation design. Thus, Geotechnical Engineers preparing proposals for a geotechnical exploration and report in accordance with this recommended practice must all follow certain minimum proposal requirements,which can help ensure a more uniform selection process during the procurement of their services. In requiring the use of this recommended practice,the participating Structural Engineers understand that the request for the information specified herein would most likely increase the cost of the geotechnical work, since there is no intent to delete any of the work currently being executed in the geotechnical industry. However, they should also realize that this additional cost is necessary in order for them to better understand the soil characteristics of the site on which they plan to design a foundation. When using this recommended practice, it is expected that the Client will provide a description of the foundations and structures proposed for the site. In addition,the Client should provide site plans that show the foundation outline(s), the foundation location(s), and the location and species of any trees that are planned to be removed and that have trunk diameters equal to or greater than 6 inches. if the lots are Wooded Lots, it is intended that the Client will provide a tree survey to the Geotechnical Engineer, showing the location, sizes, species and condition of the trees on each lot. The Client should not discount this requirement as something less than a necessity. Though not recognized locally to be a problem as recently as ten years ago, trees in this area are now known or at least suspected to be the main contributor in the majority of foundation problems in the local market. Therefore the recommendations addressing trees should not be taken lightly. • • FPA-SC-04-0 Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Nlebsite Publishing Foundation Performance Association-Structural Committee Page 3 of 9 This recommended practice addresses a geotechnical report prepared specifically for foundation design and construction. In new subdivisions, the type of geotechnical report that addresses the streets and utilities is not acceptable as a substitute for the work specified herein. Preferably,the borings for a new subdivision should be taken after the streets are cut and the lot's fill is compacted. If however, the geotechnical exploration is made before the streets are cut, then it is the intent of this recommended practice that a separate exploration will later be procured in order to verify the required density, moisture content, and Atterberg limits for the fill material. This recommended practice is written specifically for use in Houston and the general southeast area of Texas. Therefore, it should be used with caution if utilized elsewhere or if adapted for foundations other than those supporting residential or light commercial structures. The main purpose of this recommended practice is to bring certain minimum requirements together into one document for local Structural Engineers to use in part or in whole, as they see fit. It is not meant to imply that problems will not occur if geotechnical explorations and reports comply in part or in whole with this recommended practice. The Foundation Performance Association and its members make no warranty regarding the recommendations contained herein and will not be liable for any damages, including consequential damages resulting from the use of this document. DEFINITIONS For the purpose of this document, the following definitions apply: Builder--The general contractor responsible for performing the construction of the foundation, including the site work. Client—The person or company using this recommended practice in the procurement of the geotechnical exploration and report. Geotechnical Engineer—The engineer or engineering firm responsible for performing the geotechnical exploration and for providing a report of the results. Structural Engineer—The engineer or engineering firm responsible for performing the structural design of the foundation. Wooded Lot— A lot that contains at least one tree per thousand square feet(h per 1000 SF) of lot area, with those trees having trunk diameters greater than or equal to 6 inches. Note that the trunk diameter measurement is intended to be made at approximately chest-height above the ground level. Although the proper term for tree stem diameter in arboriculture is "caliper", that term is purposely not used herein because it is sometimes confused with "circumference"when measuring trees. • 4110 :PA-SG-04-0 Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Website Publishing Foundation Performance Associa lion—Structural Committee Page 4 of 9 TABLE OF CONTENTS 1.0 MINIMUM CRITE:RIA 1.1 General 1.2 Site Exploration 1.2.1 Area Reconnaissance 1.2.2 Borings Quantities and Locations 1.13 Boring Depths 1.2.4 Sampling Frequency 1.2.5 Field Testing and Logging 1.3 Laboratory Testing 1.4 Reporting 2M SPECIAL REQUIREMENTS 2.1 Slab-on-Grade Design Parameters 2.2 Dre,d Piers Design Parameters 2.3 Suspended Slab Design Parameters 2.4 Select Fill Parameters • • FPA-SC-04-0 Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Website Publishing Foundation Performance Association-Structural Committee Page 5 of 9 1.0 MINIMUM CRITERIA. The following subsections outline the Geotechnical Engineer's minimum requirements in accordance with this recommended practice. 1.1 General The Geotechnical Engineer should provide the Structural Engineer and Builder with sufficient information to enable them to: (a) design a structural foundation that is appropriate for the site conditions and is capable of adequately supporting the given building design loads, (b)provide a safe foundation design that meets local code and professional standards, and(c) carry out the site work and foundation construction in a safe and efficient manner. The Geotechnical Engineer should advise the Client if any requirements herein are in direct conflict with local codes and professional standards. In addition, the Geotechnical Engineer should carry out the work and prepare the report with the assumption that the Structural Engineer will never actually see the site for which the foundation will be designed. 1.2 Site Exploration It is the Geotechnical Engineer's responsibility to investigate the site as required to comply with local codes, professional standards and the requirements of this recommended practice. In addition, the site exploration should include the following minimum criteria: 1.11 Area Reconnaissance The Geotechnical Engineer should check the area of the building site and the area surrounding the site for any anomalies such as streams,ponds, fill, dumps, existing above-grade structures, escarpments, slopes,poor draining areas, seeps, outcrops, large trees, tree stumps, erosion, structures, roadways,railways, areas that appear to be wetlands, or anything else that will help the Builder and Structural Engineer understand the prior and present land use of and around the site. Representative color photos of the site should be recorded at the time of the site exploration and should be included in the final report. Where possible, the photos should include portions of the properties adjacent to the site. All anomalies, including trees and tree stumps with trunks equal to or greater than 12 inches diameter should be located on the boring plan. On Wooded Lots the Geotechnical Engineer should superimpose the data from the Client-supplied tree survey on his plans. 1.2.2 Boring Quantities and Locations a. For new residential subdivisions that are anticipated to have grade-supported foundations, at least one boring per 5 lots is recommended, but not less than one boring every two acres. b. For new residential subdivisions that are anticipated to have pier-supported foundations, at least one boring per lot is recommended. c. For individual residential lots or for properties with light-commercial buildings, one boring for every 2,500 square feet of building ground floor slab, but a minimum of two • • FPA-SC-04-0 Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Website Publishing Foundation Performance Association-Structural Committee Page 6 of 9 borings for the Iot. The borings should be taken inside the projected building perimeter or as close as possible, if obstructions exist. d. For lots with predominately cohesive soils and with trees growing within 25 feet(even if located on adjacent property) of the proposed foundation, one boring should be taken within 10 feet, or as close as possible, of the largest tree having a trunk size equal to or greater than 12 inches diameter, even if this dictates an additional boring for the lot. e. For additions of less than 1000 square feet, one boring is adequate, except that the boring should be taken within the proposed foundation area and the recommendations in Paragraph 1.2.2d are still applicable. 1.2.3 Boring Depths Boring depths below are measured from the grade existing at the time of the site exploration. a. The minimum depth of every boring should be 20 feet. However, if the upper 10 feet are predominately cohesionless, then the minimum depth may be reduced to 15 feet. b. On Wooded Lots, or lots containing one or more trees with trunks equal to or greater than 12 inches diameter, and if these lots contain predominantly cohesive soils, borings should be a minimum of 25 feet depth. c. For sloped lots, where the proposed foundation is to be situated at a height H above the toe of an embankment(or estimated toe if submerged), if the horizontal distance from the foundation to the toe is less than 4H,then the depth of borings recommended should be a minimum of 2H. 1.2.4 Sampling Frequency a. Undisturbed samples should be taken at a minimum of l-ft, 2-ft, 4-ft, 6-ft, 8-ft, 10-ft, 12-ft, 16-f1, 20-ft depths, and thereafter at a maximum of 5-ft intervals. b. If fill is known to have been placed on the site, sampling frequency should be increased to one sample per foot in the gill regions. c. A sample should also be taken at the bottom of the borehole. 1.2.5 Field Testing and Logging a. Each sample should be visually classified and logged during retrieval. b. Existence and depth of roots should be noted. c. Hand penetrometer testing should be done and reported on all cohesive samples. d. Standard penetration testing should be done and reported on all cohesionless samples. e. After the borehole is complete,measurements of the free water surface should be made and logged at completion of the borehole, and then again upon completion of the sitework, with that time interval being reported. 1.3 Laboratory Testing The Geotechnical Engineer should perform sufficient laboratory testing to comply with the requirements of standard codes and local practices. However, the following laboratory testing is recommended as a minimum: • ! t PA-SC-04-O Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Website Publishing Foundation Performance Association-Structural Committee Page 7 of 9 a. Existence and depth of roots or root fibers should be observed and reported for each soil sample. b. Moisture contents should be performed on all samples retrieved. c. In cohesive soils, Atterberg testing should be performed on a minimum of one third of the samples retrieved, with emphasis towards the upper strata. Reports should include plastic limits, liquid limits, and plasticity indices. d. In cohesive soils, the percentage of clay(minus 2 microns) should be tested using a hydrometer. e. Cohesive samples at the recommended foundation bearing depths may be tested using the torvane testing device provided a baseline test is made using unconfined compression tests for comparison. f. For lots that have predominately cohesive soils, testing at each boring location should be done to determine soil suction. Soil suction tests should be conducted using the transistor psychrometer method, the filter paper method or other methods that give similarly reliable suction values. Where suction testing is recommended, it should be done at sample depths of 2-ft, 4-ft, 6-ft, 8- ft, 10-ft, 12-11, 16-ft and 20-ft, but may be terminated earlier at the depth of constant suction, if determined. 1.4 Reporting The Geotechnical Engineers may use their own standard reporting techniques. However, the final report should also contain the following where applicable: a. A statement confirming that the geotechnical exploration and report are in accordance with the requirements of this recommended practice. If any exceptions are taken, the report should note each exception and the reason for taking the exception. b. A general description of the site and surrounding properties, specifically addressing the anomalies as discussed in Paragraph 1.2.1. c. Color photos of the proposed building site and where possible, adjacent properties. A minimum of two photos is recommended, but the total number of photos should at least be equal to the total number of borings. d. A plot plan showing the approximate location of borings, tree trunks equal to or greater than 12 inches diameter and all anomalies as described in Paragraph 1.2.1. In the case of trees, include species where known,the condition of the tree if not healthy(i.e., "dying,"or"dead") and show trunk diameters, measured at chest-height. If the site personnel are unable to identify the tree species,then an attempt should be made to classify them into categories that help the user to estimate the potential water usage of the tree. For example a tree could be classified as either a hardwood or pine. Alternatively, it could be classified either as a broadleaf or conifer. e. Boring logs that include all field and laboratory tests results, unless particular data is presented on other charts or tables. E Descriptions and classification of the materials encountered. g. Elevation of the water table, if encountered. If no water was encountered, the report should state that the holes were "dry". h. Provisions to mitigate the effects of expansive soils. i. Recommendations on earthwork stabilization requirements(including requirements for slope stability) needed to prepare the site before the foundation can be constructed. • PA-SC-04-0 Recommended Practice for Geotechnical Explorations and Reports 11 April 2001 Issued for Website Publishing Foundation Performance Association—Structural Committee Page 8 of 9 j. A discussion on foundation maintenance required in order to maintain the design. k. Combined(i.e., for the various borings)plots of moisture content profiles and plastic limit profiles vs. boring depths. 1. Plots or tables showing the percentage of clay in cohesive samples as determined from hydrometer testing. m. A discussion of the degree of saturation or desiccation of the site as compared to the estimated equilibrium moisture contents of the samples. This can be presented graphically depending on the method used (e.g., a graph of moisture content minus plastic limit vs. depth). n. Combined(i.e., for the various borings)plots of suction values vs. boring depths. o. Interpreted output from the suction testing including the moisture active depth, the movement active depth, the edge moisture variation distance and the probable vertical movement, both up and down, of the ground surface. p. Specific discussion of trees to be removed before construction and of trees that are to remain after construction is complete, if known. 2.0 SPECIAL REQUREMENTS In addition to the general minimum criteria discussed above, there are some specific requirements that may be applicable to the Geotechnical Engineer, depending on the Client's needs. These requirements are as follows: 2.1 Slab-On-Grade Design Parameters Regardless of the type of building foundation planned, if design recommendations are provided for slab-on-grade foundations, then the Geotechnical Engineer should provide recommendations as outlined in both (a) W RI's "Design of Slab-on-Ground Foundations," latest edition and(b) PTI's "Design and Construction of Post-Tensioned Slabs-On-Ground," latest edition. 2.2 Drilled Piers Design Parameters If design recommendations are requested or made for drilled piers such as those recommended for slab-on-piers (at grade), suspended(structural)slabs, or structural floor-on-piers (i.e., with a crawl space) foundations, then the Geotechnical Engineer should provide recommendations for pier depth that takes into account possible upward and lateral movements as well as the normal downward movement due to gravity loads. Upward movement should be addressed if the soil is predominately cohesive. Lateral movement should be addressed if the site has pronounced slopes or if substantial fill is planned or has already been placed. In addition, the Geotechnical Engineer should also provide similar design recommendations as specified in Paragraph 2.1. Further, allowable design loads and recommended depths should be provided for both(a) drilled and under-reamed piers and(b)drilled straight shaft(skin friction) piers, in order to give the Client an opportunity to perform or obtain a cost/benefit study. 4 • �� �� = STATE OF PRACTICE OF GEOTECHNICAL ENGINEERING FOR DESIGN OF CUSTOM HO.IVIE3IN THE HO usToN AREA BETWEEN 1990 AND 2001 BY BY DAVID A. .EASKWOOD' P.E.' BY FRANK ONG, P.E.: PRESENTED AT FOUNDATION PERFORMANCE ASSOCIATION MEETING ON JUNE 20, 2001 Abstract Practice of geotechnical engineering in the Houston area has been quite interesting during the past decade, depending on the firm, recommendations for design of custom residential homes vary quite a bit. The purpose of this paper is to look at various design approaches and recommendations. This paper summarizes the state of practice for the past decade. Furihcrnmore, the paper recommends procedures to conduct better geotechnical exploration for custom residential projects in the Houston area. Introduction Due to the strong economy, custom homes are being constructed all over the Houston area. Most of these homes are supported on drilled footing type foundations. The focus of this paper is primarily on the dnoigo of homes on drilled footings. Many odd shaped homes (lJ. I. shaped or houses with large slabs with notches) are supported on drilled footings. Fortbernmore, some of these houses have major foundation problems. The purpose of this paper is to review and summarize 99 geotechnical reports in the Houston area, look at various soil types, discussion of riokm, heave computations, drilled footing dep|&s, various slab desi s, evaluation of environmental conditions, etc. These reports were conducted by 17 different firms located in the Houston area. About 10Y6 of these reports were conducted by Geotech Engineering and Testing. This paper also develops recommendations on how to better conduct geotechnical exploration for custom residential projects in the Houston area. Report Research ---'---- In order to develop a State of Practice Report, research was conducted tofiudgeotccbnjco| reports for custom homes conducted by various Geotechnical firms in the Houston area. A total of 99 reports were located from GET's library and from various structural engineers throughout the Houston area. These reports were used as a basis of development of our findings. A map showing where these various soils reports for custom homes were conducted is shown on the site plan, Plate 1 of this paper. As indicated on this map, the concentration of these reports are located within 610 Loop area, near West L/oivrcsity, Bellaire, Medical Center, and Kirby area. Fmctbermnre, some of these reports are located along Memorial Drive, Hunters Creek Village, Piney Point Village and Bunker Hill area. A few data points are located outside the Loop. Y,iucipal Engineer,Vc^te"i` Engineering and Testing, 880 Victoria Drive,Houston,Texas 77O2Z.7|3-699'40VO 2 Chief Engineer,0eoroch Engineering and Testing, 80U Victoria Drive, Houston,Texas 77O%2^7i3'69Y'4000 1 of 1| S • Definitions The data developed during this research study are summarized on Plate 2 of this paper. The definition of each term used. in this paper is presented below: Map I.D. - The map 1.D. indicates where a specific soils report was conducted. The map I.D. is key to the site plan, Plate 1. Year - This indicates the year the soils report was conducted. Company This term identifies which company did the soils report. We have companies A, B, C, D, E F. G. Fl, .), K, L, M, N, 0, P, Q, and R, a total of 17. Report No. -- This term designates the report number for each specific company. Expansive Soils - This term signifies whether or not expansive soils are present at the job site. .. Expansive soils should have minimum plasticity index of 20 (Ref. l). Trees on Site/Site Conditions - This term signifies whether or not trees were present on the site or the firm who conducted the geotechnical report had a Site Condition Section in the report. Sometimes, these soils reports did not even discuss the site conditions. In this case, a dashed line is put in this space for the segment "trees on site/site conditions". Effective Plasticity Index (Pf) - This column presents the effective plasticity index of the soils developed by BRAB method (Ref. 2). Discussion of Expansive Soils - This column signifies whether or not the geotechnical report discussed the presence of expansive soils on the site. Discussion of Risks - This column describes whether or not the geotechnical report discussed various risks that are associated with different types of foundations used for residential foundations built on expansive soils. For example, a structural slab with void would be low risk foundation. However, a slab-on--fill pier foundation will have a higher risk than a structural slab with void type foundation system. A discussion of foundations and risk is given on Plate 3. Heave Computations - Most of the heave computations for residential projects (if computed at aft) were computed by using Potential Vertical Rise method (Ref. 3). This column signifies whether or not a heave computation was computed for the soils report. 2of1i • • ;.-gilled .Footing Depth - This cotrwrcmr3 signifies what was the recommended aver depth for the specific soils report. Structural Slab - This column signifies whether or not recommendations on a low risk foundation, which is a structural slab with voids/crawl space, was given in a specific geotechnical report. Furthermore, if recommendations on structural slab was given, whether or not recommendations on voids under the floor slabs was given. Therefore, this column signifies whether or not recommendations on structural slabs were present and if recommendations oin structural slab were present what was the recommended void size under the floor slab (not under the grade beams). Slab-on-Fill - This column defines whether or not recommendations on slab--on-fill were given in the specific geotechnical report. Furthermore, whether or not a specific fill thickness was given. Some reports presented a very vague fill thickness recommendations. Void Box - This column signifies whether or not recommendations on void boxes were given under the grade beams. Drainage - This column signifies whether or not recommendations on site drainage around the house were given. Sprinkler - This column signifies whether or not recommendations on the presence of a sprinkler system, and their location around the house were given. Trees - This column signifies whether or not recommendations were given on planting a tree next to the foundation. Fin t rermore. it indicates whether or not recommendations were given on the existing trees next to the foundation. Tree Root Removal -- This column signifies whether or not the specific geotechnical report discussed how to treat the tree removal from a specific site. What would be the ramifications of tree removal (heave). Construction Monitoring - This column signifies whether or not the geotechnical report gave specific recommendations on quality control such as conducting of testing including drilled footing observations, concrete testing, earthwork testing by the design geotechnical engineer. Review of Foundation Drawings This column signifies whether or not the geot.echnical engineer of record required that for the foundation drawings be reviewed by him/her to make sure his/her design recommendations are properly interpreted by the structural engineer and other design team members. 3 of I I • • Soil Variability This column indicates whether or not the geotechnical report indicated that the soils across the site could he variable (from a standpoint of stratigraphy or properties) and there might be a need for design modifications if different soil conditions were encountered. Ar aly_s s of the Data Cenral. The contents of all of these reports were reviewed and analyzed. The specific recommendations on analysis are presented in the hallowing sections. Expansive Soils. Many of the reports acknowledged that expansive soils were present on the site. In general when the effective soil plasticity index is above 20, expansive soils are present on the site. Trees on Site/Site Conditions. About 64% of the reports did not discuss site conditions. Specifically, they did not discuss whether or not trees were on the site or any•other site features were located on the property. This is an extremely important part of a geotechnical report where many firms failed to discuss. Effective Plasticity Index.-- About 82% of the reports discussed how expansive the soils were at the - specific site and gave plasticity index data in the report. About 91% of the sites reviewed had expansive soils on them. Discussion of Expansive Soils. About 82% of the soils report reviewed did have a discussion of expansive soils within the body of the report. Discussion of Risks. About 40% of the reports did not have a discussion on risks of using different types foundations. For example, they did not discuss whether a structural slab was better than a slab-on-fill type foundation supported on piers. Again, this is extremely important because, by discussing the risk associated with each different type of foundation, the soils engineer brings in the architect, the structural engineer, the builder and the owner into the decision making process. Heave Computations.ions. About 74% of the reports did not discuss or calculate the heave. It is not customary to estimate the heave for design of custom residential foundations in the Houston area. One of the reasons for that is, it's not a consensus on the correct method for estimation of heave. Furthernore, it could. be expensive. Therefore, most geotechnical consultants use their experience in specifying how much fill is required under the floor slabs to reduce heave on various types of residential foundations. The required fill thickness is usually determined based on the experience and engineering computations of the heave, such as Potential Vertical Rise, PVR (Ref 3). Dri ledF ooting Depth. An average pier depth of about 9-ft was specified after reviewing all of these reports, Review of reports from 1996 to 2001. indicated an average pier depth of 10.5--ft. Depth of drilled itootings is very important in areas where expansive soils are present. Shallow piers can push up against the grade beams and lift the foundation system, if expansive soils are present at the site. The pier should be placed below the zero movement line. The zero movement line is a line below which no movement (heave) of expansive soils occurs due to weight of the colum of the soils. The piers should he anchored below the zero movement line. Currently, we recommend piers to be placed 12-ft to 15--ft in the Houston area where expansive soils are present. 4of11 • • Structural Stan Recommendations. About 55% of the reports discussed structural slab systems. The other 45% did not discuss at ad. Furthermore if they were discussed, most of diem did not specify any kind of a void space that should go under the structural slab system. We believe lb-at there should he a detailed discussion about the use of a structural :slab. Furthermore, the recommended void size should be specified. In addition, recommendations on venting of the air underneath the slab should also be discussed. To limit moisture migration through the slab. This is only applicable to a structural slab with a void/crawl space. Slab-on Fill, About 97% of the reports did specify a slab-on-fill type f-oundation or drilled footings system as the type of foundation that should be used to support the structural loads for a typical custom residential. foundation. Some of these reports were vague, because they specifically say whether or not fill is required under the floor slabs. The vague statements given did not specify exactly how much fill should he placed under the floor slabs. Majority of the soils reports reviewed indicated the required Fill thicknesses of about 24-inches or less. In general, a maximum of 48- inches of select structural fill was specified in the areas where the soils were highly expansive. Our experience indicates that about four-ft of fill will generally reduce the movements to an acceptable level, provided positive environmental controls (drainage, trees, sewer/plumbing leak, etc.) are implemented. Void 'Boxes. About 38% of the reports specified the required void box size under the grade beams. Void boxes are recommended by many geotechnical firms in Houston as a way of reducing foundation movement ":xpansive soils once swelled up can theoretically move into a void space area (void box) without lifting the grade beams. The discussion on whether or not void boxes should he used under grade beams on residential foundations was conducted by the Foundation Performance Association. it is generally believed that void boxes under grade beams provide channels for water to flow underneath the foundation system. Therefore, the use of them are discouraged. This discussion and idea was developed in 1996. Drainage. About 93% of the reports discussed that positive drainage was extremely important to the performance of the custom foundation system. Drainage was discussed in a majority of the reports. Sprinkler Systems. About 86% of the reports reviewed did not discuss the sprinkler system. They did not discuss how the sprinkler system (if used at all) should be placed around the structure to minimize moisture variations and therefore, differential movements. Trees. About 59% of the reports did discuss trees. Specifically, all the reports that discussed trees, described planting trees next to the foundation system and how they would affect the foundation system. It is understood that if the tree is left in place or planted next to a foundation system, it may cause the soil to shrink and the foundation to settle it' clays are present. En all of the reports the word planted was synonymous with trees left in place. Tree Removal. About '/0% of the reports did not discuss the tree root removal in their reports. This is an extremely important section of a report, because tree removal in areas where expansive soils are present, can cause significant heave. Therefore, the reports should address this condition and warn the client. This has not been customary in the Houston area until the year 1995. A detailed study of tree removal and its effect on foundation systems was presented in 1997 by Eastwood and Peverley (Ref. 4). 5ofII • • Construction Monitoring. About 85% die reports suggested following up the design with construction monitoring. Construction monitoring in.g is an important part of any design. Review of the Plans and Specifications. About 75% of the reports suggested review of the foundation drawings after the design was completed. The review of the foundation design drawings by the geotechnical engineer of the record is important. This review will provide the client with the confidence that the initial designer (architect. structural engineer, owner) understood the soils report and followed the recommendations. It is possible once the drawings are reviewed by the geotechnical engineer of the record, mistakes are found that are reported to the structural engineer. Furthermore, foundation and risks are discussed. Soil_Variability_ About 99% of the reports discussed the potential variation of soil stratigraphy and properties across the site. This is a true condition. Subsoils may vary across the lot from a standpoint of stratigraphy and soil properties. Conclusions and Recommendations Based on the review of the 99 reports written between 1990 and 2001 for custom homes in the Houston area, the following conclusions and recommendations can be made: o The soil reports reviewed from 17 companies represent a cross section of the geotechnical firms in the Houston area that do residential work for custom homes. o Many of these soils reports indicated the presence of expansive soils in the vicinity of the project site. Most of them discussed the presence of expansive soils. o Only 36% of the reports reviewed discussed site conditions. The rest of the reports did not even address site conditions. The site conditions should he discussed in all reports. Perhaps a picture of the site should be included in the report. o Foundation types and risks must be discussed. o A review of the reports indicates an average pier depth of about nine-ft. Pier depths have been increasing in depth in the Houston area since 1995. in the 1970's and early 80's, piers were placed at a depth of about eight-ft. However, due to new understanding of the active zone b depth and the effect of trees on foundations, deeper piers have been recommended. However, most geotechnical firms in Houston are not taking account the effect of tree removal in their foundation system. This condition was known to the Houston area after 1995 or 1996. Furthermore, the use of deeper piers is resisted by some designers, builders, owners, etc., because this may add some to the cost of the construction of the foundation system. However, we believe that increasing the depth of the piers by a few feet, the cost of the foundation system should not increase significantly. Furthermore, the risks of putting shallow piers in the area where expansive soils are present are too great. These shallow piers can actually be grabbed by the expansive soils and be pushed a gainst the foundation system, resulting in floor slab heave. Considering that most of the distress of the newly constructed foundations in Houston is heave, the piers should be deep enough to resist uplift due to expansive soils. 6 of II • • • o Almost all of the reports discussed the slab-on-fill on drilled footings. However, many of them did not suggest the required till thickness. We believe that if this type of foundation system is recommended, the fill thickness should be clearly defined. Unfortunately, some soils reports are very vague about the required fill thickness. This has caused foundation problems, because inadequate fill thickness has been placed underneath the floor slabs. o About one-third of the reports did discuss the use of void boxes underneath the grade beams. Many of them did not. The use of void boxes is a controversial issue today. We do not recommend the use of void boxes under the grade beams. o Nearly all of the geotechnical reports discussed positive drainage as a major component of foundation design in their report. This should be covered in all reports. o Not very many reports discussed the presence of sprinkler systems around a house. We believe that the sprinkler system, if used, should be placed all around the house to provide uniform moisture conditions at the edge of the foundation. A non-uniform moisture condition will result in differential movement of the slab and foundation distress. o The area of the geotechnical reports that requires most improvement is the section that has to do - with removal of existing trees. This issue must be further discussed in geotechnical reports. Currently, most geotechnical reports in Houston do not even discuss the effect of tree removal. l.ri the event that a drilled footing foundation system is to be used, minimum boring depths should be 20-ft. Root fiber depth should be logged in the borings. Furthermore, the depth of active zone should be estimated. The effect of tree removal should be clearly discussed in the report. o Structural engineers who are designing a custom home must make sure the effects of tree removal have been considered in the geotechnical report prior to conducting a design of a residential slab. Some structural engineers blindly disregard this issue and they claim, they followed an erroneous soil report. Knowing the site conditions is also the responsibility of the structural engineer of the record. o None of the reports reviewed had any suction data or used suction to estimate heave. o The authors hope that in the future, the concepts, such as the use of suction will be implemented in the design of lightly loaded structures. Currently, most firms in the Houston area do not run suction tests on their soils samples due to costs associated with conducting this type of test and analyzing the data. Due to the extremely competitive nature of doing residential geotechnical work, it is almost impossible to do a detail geotechnical exploration and testing for residential projects. We believe that by providing a minimum standard in conducting geotechnical explorations, more firms will be interested in conducting more advanced and up to date geotechnical explorations and therefore, saving the client from the risks and spending too much money during construction and repair. Having a minimum standard is the only way, we believe, that the practice of design of a custom foundation system for lightly loaded structure in Houston can be improved. 7 of 11 • References . David Eastwood and Others "Methodology for Foundations on Expansive Clays" Published in December, 1980 Edition of A SF E;our nal of Ceott c hntcal r:n;ineer n- Division. 2. Building Research Advisory Board, National Research Council, "Criteria for Selection and design of R srth cia1 Slabs on Grounds," National P catierny of. Science Publication 1.57 f 19o8, 3. "Method fb. Determining the Potential Vertical Rise, PVR," State Department of Highways and Public Transportation, Test Method Tex 124-F, Austin, Texas. 4, D. Eastwood and D. Peverfey "Design of Foundations with Trees hi Mind", presented before the -'-.SCE, Texas Section, Spring Meeting in Houston, April 1997. 8ofH • 0 i .-=' A t t IV . , _.-.. ,/ „ C. .-•-.% < tia% , 1 \''' I,. .4 ': _ .......—■' %' ' . i I' - ' _., ,14'.'',,......,J j --- —.--...1_, ; ;N,:,J,tN,-1..:, - ' -...,,- —., E - .r, .,,,.... t— ' 4 ,Z4 .i• -74 '--.,,, 1 ..... .., I ....IL; .1,-..‘ . . ,serk.1 .6. . . • . . - P •,,,h, 0 ---......2 .z. e't %., ,- ,;'' •,.- ., , _..../y ..... \ ;,.. , ,,,,t ,,!,„... Iv -,I ' , ,,.„.„ ,... _,,,„ • i ..... • to---;,,,,,,,, /."--- t -- 1 l...,N.., 4, V ?: , 14 , ,-;:z,..°:... ...----- •--,,,,1<:- ..„ ' ; — 1 41: -h-,- . „sc.t: 7....44, -...-hi . • ,,.■ . 9."«. •,*-4 1, ,Y.0P-'7--X."- •=. . 1 •-•,-;-, . ..4,,,i' 4 0 ,.-{ .., ' , ''''' '4. ' 34..''' (.1 '--hult ,14:57-A------ A-,_ ' k ti q ,h, , ••• •••„_----.. — :..Khl, ...„, it......- a........, irl 'er---- - , 1 i l'i,"- t3) .... , r\-, ....,;'' '---,...,,, • `-. '"------- 6 g 11 AL4I1-, J h -1 _. ....„.....„. , .i. , ,.17.-6,-. .,....._ 4 rr N . ,, V 7., ';: V. :. 1 '17..:,,-f-7-----it------------7-#. • 11---1-47.,- ;-..°4..I.,r. C7:3::,IA :::: ' '"icIZt_.1 '-'4 --!..: ...,.........4 .., _ 11 1 ,f.');k, At.- ---)...,k,ri.,0...x - : •th-.-zi-Akhh„,,,, -iy, i'.7_ .--- , h. ' .i,. r...h •,,,h- . A..,...''',,e a......-4, -... ... 1 i 1 7a-•: ' al iii i ctr,..-." Ot a ''•,/,‘1."..'.,,v,..y,>„a,.,,,,. Z','*..-,*-.,,_..,,,I A j_.k2 „,4...,i-,,,,_...,,,„_ i, _,..,,...rpe,*,. ,7..., -. ,' 1k P.A , .,-N..., - " al a c '44 r”,h it - • -* i N'sjij ' V I V' ''''''.V1?"51.' . _ iri - , ,. ..: gy- (-1. — - ----_, "`• - - __:-.-_ .7" iv/ lilt (673.1-- cid 1 i yti::....", --- 1--4:illi -2-; "..." ,;i• e.f• d'''' / '7,7-...:=1111 ; li -I...._ ,• ‘ • ......A ---. ,,... .= ,.. 1 ii 75, -Ai ';' ,, w...-,,jeeeThe -_-_-, i1 ' ::\ T., LCV: ee`- 1' ',Or- af St.1 k a ^ • ■••••,....e „T,,L.,,,,..__Lx..\,..,.,1' . ,__, - -- , ... NS. . .... ...- 1W I ----- . 1 ,..-:,,,sm., N,. •si -, . ,-, 1 -4,1::: A c-1 • --af. I -,'*. I , r ■ ... . ;" ; .4 ; j e ;,-------7 1 1,--L...„ 1, ...... . . ,.6 2 Lii ....:\ i: 1 , e J ' I _7....... -...A ,„.„ 1 ', q 4ntu .,.. Ni 1 t,1 t . d.4 ''.':,,, i ,1- f i I .-%. ,i, Site Plan Plate 1 9 of ii • STATE OF PRACTICE,RESIDENTIAL FOUNDATIONS 1990 THROUGH 2001 _ Rego d 1 Eiya IT a ER tie Ci - D'xurs He;rve fJMled Sntr:Nrai^dah lab an FN Void Tree Ca Sln Ra»ew of Soil Map Ropot on Pk' alv o!E 1 ion (:on (Fv-1 xj�R �Deal N R' e F-Il Roe Drain R rue lion Fotndelum Vann ICI Year C. No. 040 See? Mtl PI Soll 01 Risks tarn D In,11. Miens ,re tl Tisanes, C - lterc: Remove; hondon• °mein•s IA "i 1990 Jtan• 209901E Yes - 3) 0W No No 0 Nos Yes 12 n. ,v4 tn.®® No No lien K 901025 Yes 0�� V'es ® _ �j11:/®IL1� Yes No ��r__ H G0I-330 Yea Yos N� Y. I �y-e.1����p1♦ Ves an B 9161026 Yes "yes' _ NEU H 6.91-0063 Yea ®� Na No - T. 16 �� Y. 1!1!11 N 0611-157 Yes MI NW No _ No MM�NMI Yas _ �� � � 1�1 H G81-010 Ves Yes Yes _0 ® L 191031 Yes =� Yes No 0 �© Y s 24 0®�®. Ves Y. 9 1!X11 H 691 125 Yes Yes Yes 9 Yes - ' I Yes Yes Yes Yes Ion 19 iii 14 289-01239, Yes -- _ Yes No Yes Yes 11 I,N1 H 091-252 Yes ��Tns�— Yes Yes 0� . Yes 214 Yet a Yes Yes Yes Yea Yes 12 1991 L 191014 Yes 40 Yes Nr. 8 -- Yes 24 fi No NN_ No N'o No Mill 13 1991 G —9101om No Yes 15 Nn No No - Na - Yes - _ Ves Nn N0 No No 14 1991 P HE-01-077 , Yes -- 60 Yes Yes No 10 N0 -. Yes 24 - Yes Ves- Yas Yes N li 15 1932 A 92-159E Yes Yes 35 Yes Yxs No 10 Yes 5 Yes. 24 4 Yes Yes Yes Yes Yes ® 1(412 B 927/2020_ Yes ®® Yes yes Na 9 r Yes 1624 ®® es es a No 17- 19!12 G 9211-2025 Yes a 60_ _ Yes Yes Yes a Vxs' Yes 214 �® Ves Yes 111_ 1192 H 692-243 Yes a 65 _ 19 i!h'2 K 92.11170 Yes 25 Yes_ No b 'Yes Yes 15 No No 1.12 116920 Yes arm No ® ! ®, S Yes 12 a� ®, 1992 692-210 Itt, ®� ��® I'�' ��® 11111111111311111111102•111111111•111111111111W Yes 1!192© 91.18-1 Yes ®E Yep: - ��®® 992 H 002-209 Yes ®�® - ®®���� 1192 .1 100-92E Yes ��®®0®� Tea 12 a�©®���Y. 152 G 9trees11 Yes Yee Yes No ®�� Yes 18 aten®�, ® - 1902 L 19205) _ Tee Vas 140 _ Ni, Too 24 M 4tr����f� 1992 J 2iF?92E Yes ®� No 140 Na �� - Yes _ 12 �® F 92040-1 V. =® Yes a0s Yens I 1111111111122111211��� ® M 009112 No 19 Na �� Yxs - ® 112111111211111111E1111111112211111 Yes-- 30 I992 J 210-92E___ Yes E. A2 Yes - Yen 24 ®��� 31 199-0 V. 92-2560 Yee ® 32 Yes �� Yer. 20 ® 10, H G92-583 Yes a 51 Yes =151 -- ®���, �® 1. 1992 M 623.92 Yes Yes �� Ves Yes _ 12 19112 H ,92-122 yes ® 51 Yes __. Ye.. 993 C: 931040/1 Yes 38 - Yos Nn Yes 24 gagB 9363867 Yes ® Fes Nn ® -, 0m; _ 24 � �� 1!193 H 1303211 Nn -- �� 0 e9 Yes - V.-s ��� � 30 1 E '1994-1 Yes No �® No Yes ®ll - Yes - Yes ®��®® 39 993 9 9203274 Yes fit O. Yes NO S Yes _ Y 0 4 ® Yes 40 1993 i B 9303921 Yes M 48 Y Y s No 0 ' Yes -- Yes 1_. 4 rlteMINM - 41 1993 B 936.1565 Yes ® 40 -- ��®' ®' ��� 42 ' B 930401,2 Yes =� Tel No NM - Vns Ill-la UMW= m�� 43 15[13 No No H No _- Y ®m��� Yes 44 15'73 0 9303471 Yes Yes No © Yes ___ IB ���® - 4. 1993 0 9311-1021 Yes ® S6_ Yes No ® 1. -- Y. _ 24 46 B 930'.1609 One ®' 32 ilin Yes No ®® Yes 24 11:11111=11112111M, 1117111M111141 • 41 1903 J 277;930 Yes 4y �M�� Ens - • ���® � 40 Inpy J 28493E Ves 111111•1111111® Na No. Yes - araimmamais.���®_ 49 1993 J 118_93E Yes ®= N N.— No_M� Yes__....—_ 12 A 3 J 127 930 Yes 5f 199 M 14_93 Yes 32 Yes No Ye_ 12 �M � -52 1993 J 224203E Yes M 25 = No Na �� Yes _ 12 53 El N 265-75033 Yes a 49 ® Na Yes, a — - _ 24-36 111111111121111111112N112111110111111111311111® E® F 93802-1 Yes 28 Yes Na I No Yes _ ®® Nn No ® No E� C. 931(.11 yes ® 42 Yes Yes No ® Yes Yes 36 Yr Vxs Yes 1993 A 942347E Yes Yes 42 Yes Yas No �� 6 Yes 'A 5t 1994 B 941;4517 Yes ® 55 ®�®���a_m_ a ® H 694-959 No ��®� Yes _ �����® ®59 B 046-0 Yea ® 1-.0 Yes 18 ®' ®�� 1994 0 94'4-1003 Yes � -- ���, 6 .. 'Yes 18 3 976 F 947331 No - ® Yas No No J Yes - No No No 954 0 9464519 Yxs ® e 'e --._ - ���� ®®No N -' ' ' - 1994 0 940451H V. N° Vb.. Yes 1!194 B 946011 Yes ®. ®11111211• Yes 18 lEIIIIEIIIEIIIIIEIIIIEIIIIIIIMIIIIMIZMIISEII B 9405066 Yes Yes Yes Yes Yes .. Y 4 a11® Yes V v �NEIR FM H 694103 Yes - es .. 1994 II 091-478 Yes B® Yes Yes Yes ® ves Ves ®® Yes Ves Yes Yes 1994 B 9404720 Yes Yes No Yes .- Yes 24 4 Yes N Yes Yes ®1994 IIIIMIERIEINIZIEINIZI111111211111® 9.10 ®101111 Yes a110111112111111W111 Y. MINIM 14 1004 E 9401214 Yes mIIIMIIIIIIIIEIIEIEIIIIIIIEIII U) En Yes I6 110111111,11111211 Yas No yes t 1994 A 946034E Yes -� �� Yes _ 35_�� Yes Ves Ilraillaraa 11 1995 E 94-934E Yes Yes Yas No WEIS ® 1905 8 0566119 Tea 0® ®�® Yxs 24 ®®���� 1996 B 9958076 Ves ® •®11CWIIIIIIMII® • Q®®IMEMIEMINMEMIESI 1995 F 9574Y1 Yes ®®® ®1111113•1 -- 1111E1•11111191•1111111111112111®1011111101110®112111 ®® F 051184-1 Yes ®® Yes No Yes 0 Yes. •• ® H 005221 Yes e® Yes_ 8 -. Yes I ®® fI1:C7 1995 E 9561406 Yes a 140 a® Yxs - - �nn®�� -995 A 95 4546e Yes a� Y. - "es — 36 �® ® 5 916%1;;7 Yxa �. .- Yes _ 26 4 1996 H 6901205 Yes ® - Yes - - Ves 1!018 ._-g� ��®' "e• ��� Yes am F 9(')i 13' Yes EWVII No Oar 0 Ie -- Yes 12 E N. ® = ®®® 1905 F 0603'1 Yes IMESEIN ME No Yes e ® Yes 17 N No No No No �� 996 H 6901'97 Yes IIIMIBIIIIIFMIIIII N. No e n _ Yes ®® INISNI11112111111111211 1095 A 96658E Yes Yea No 3_ � ���I�1�4m ®1197 E ;1'163010 Yes - P' Yes' No �® a�� - ®�4ti� 64 2SA 75159 Yes - ®® Yee Yes MNM A 01-410< Yes Yes V.s Nn tu� 0 �� �SZE ��� A 8-01660 Yes ev - i 4R® F 492351 Y Y.s /s Yos 9 Yxs 24 Ves Yw: ® Na 1999 E 9906440 Yes Yes Ves No 0 N 30 Nn Yas No Yes No 1!199 E 99650//0 Yes Yes Ves No 9 Yes 24 4 No Yes Nn Yes Yxs 999 E 9965225 Yas Ves Yea _No a No 24 - No I Ves Na Yes No !X9 A 91!•'040 Yxs Y.. Ves No 12 Vxs J A9 Yes Yes Yes Yes Yes C 021-137 Oss No No to No .. Yes - - ®® No Na ��® ?000 0 2K-04 Na a Yes Yes Ne =� 4 __ N. No ®�® vb 2000 A 014022E Yes � '/es Yes No � Ye:: Yas 99 ® A 01.009E Yas M� Yes Ves No 15 8 ®�®® Ves Vxs ® Yes NOTE.:I.Misleadinc recommendations on heave computation and fill thickness. Plate 2 10 of 11 • • . FOUNDATIONS AND RISKS Many lightly loaded foundations are designed and constructed on the basis of economics, risks, soil type, foundation shape and structural loading. Many times, due to economic considerations, higher risks are accepted in foundation design. Most of the time, the foundation types are selected by the owner/builder, etc. It should be noted that some levels of risk are associated with all types of foundations and there is no such thing as a zero risk foundation. All of these foundations must be stiffened in the areas where expansive soils are present and trees have been removed prior to construction. It should be noted that these foundations are not designed to resist soil and foundation movements as a result of sewer/plumbing leaks, excessive irrigation, poor drainage and water ponding near the foundation system. The followings are the foundation types typically used in the area with increasing levels of risk and decreasing levels of cost: FOUNDATION TYPE REMARKS Structural Slab with Piers This type of foundation(which also includes a pier and beam foundation with a crawl space)is considered to be a low risk foundation if it is built and maintained with positive drainage and vegetation control. A minimum crawl space of six-inches or larger is required. Using this foundation,the floor slabs are not in contact with the subgrade soils. This type of foundation is particularly suited for the area where expansive soils are present and where trees have been removed prior to construction. The drilled footings must be placed below the potential active zone to minimize potential drilled footing upheaval due to expansive clays. In the areas where non-expansive soils are present, spread footings can be used instead of drilled footings. Slab-On-Fill Foundation This foundation system is ako suited for the area where expansive soils are present. This Supported on Piers system has some risks with respect to foundation distress and movements, where expansive sods are present. However, if positive drainage and vegetation control are provided, this type of foundation should perfomn satisfactorily. The fill thickness is evaluated such that once it is combined with environmental conditions(positive drainage, vegetation control)the potential vertical rise will be reduced. The structural loads can also be supported on spread footings if expansive soils are not present. Floating(Stiffened)Slab The risk on this type of foundation system can be reduced sizably if it is built and Supported on Piers. the Slab can maintained with positive drainage and vegetation control. Due to presence of piers,the either he Conventionally- slab cannot move down. However, if expansive soils are present,the slab may move Reinforced or Post Tensioned up,behaving like a floating slab. In this case,the steel from the drilled piers should not be dowelled into the grade beams. The structural loads can also be supported on spread footings if expansive soils are not present. Floating Super-Structural Slab The risk on this type of foundation system can be reduced significantly if it is built and Foundation(Conventionally- maintained with positive drainage and vegetation control. No piers are used in this type Reinforced or Post-Tensioned of foundation. Many of the lightly-loaded structures in the state of Texas are built on this Slab) type of foundation and are performing satisfactorily. In the areas where trees have been removed prior to construction and where expansive clays exists,these foundations must be significantly stiffened to minimize the potential differential movements as a result of subsoil heave due to tree removal. The beauty of this foundation system is that as long as the grade beams penetrate a minimum of six-inches into the competent natural soils or properly compacted structural fill,no compaction of subgrade soils are required. The subgrade soils should; however,be firm enough to support the floor slab loads during construction. The structural engineer should design the floor slabs such that they can span in between the grade beams. The subsoils within which the grade beams are placed must have a minimum shear strength of 1000 psf and a minimum degree of compaction of 95 percent standard proctor density(ASTM D 698-91)at a moisture content within +2%optimum moisture content. Floating Slab Foundation The risk on this type of foundation can be reduced significantly if it is built and (Conventionally-Reinforced maintained with positive drainage and vegetation control. No piers are used in this type or Post--Tensioned Slab) of foundation. Many of the tightly-loaded structures in the state of Texas are built on this type or foundation and arc performing satisfactorily. In the area where trees have been removed prior to construction and where expansive clays exists,these foundations must be significantly stiffened to minimize the potential differential movements as a result of subsoil heave due to tree removal. However, foundation tilt can still occur even if the foundation system is designed rigid. The above recommendations, with respect to the best foundation types and risks, are very general. The best type of foundation may vary as a function of structural loading and soil types. For example, in some cases, a floating slab foundation may perform better than a drilled footing type foundation. Plate 3 11 of 11